If one or both of a male puppy's testicles don't descend, he's known as a cryptorchid. A puppy's testicles should drop by the time he's reached the age of 2 to 3 weeks, although they can take as long as two months. If one or both testicles haven't descended by that time, your vet should recommend surgery to neuter your puppy and find that lost ball.
Normally, as male puppies develop in utero, the testicles are located near the kidneys. In the fetus, a cord called the gubernaculum guides the testicles to their correct seating in the scrotum. If the gubernaculum is defective, either testicle, or both, can wind up in an improper place. Generally, that's the testicle on the right side if if only one of them descends. The retained testicle generally lies inside the puppy's abdomen, between the hind legs under the skin, or in the inguinal ring, the opening through which it should have dropped.
While any male dog might experience cryptorchidism, it occurs more often in certain breeds. According to the American Kennel Club's Canine Health Foundation, cryptorchidism is the most common birth defect in purebred dogs. The breeds most affected include the poodle, Yorkshire terrier, Chihuahua, German shepherd, miniature schnauzer, Pomeranian, dachshund, Shetland sheepdog, Siberian husky, Old English sheepdog and Maltese. The brachycephalic, or short-nosed, breeds are also vulnerable. These include the bulldog, Pekingese, boxer and pug. Since this is a hereditary defect, cryptorchid dogs should not be bred.
That retained testicle can cause a puppy all sorts of problems in later life. If left inside his body, it could end up killing him -- dogs with a retained testicle are prone to testicular cancer. He could also end up with testicular torsion, which occurs if the retained testicle twists inside his body. That twisting, or torsion, cuts off the testicle's blood supply and causes the dog extreme pain. If that happens, only emergency neutering and abdominal surgery can help him.
While neutering a puppy whose testicles have both dropped is a fairly simple surgery, that's not true of the cryptorchid. The vet must find the testicle inside the puppy, then remove it. Under normal circumstances, a neutered puppy goes home the same day as the surgery. A cryptorchid puppy, though, experiences a more difficult operation and might have to stay at the vet's for a couple of days. When he does get home, his recuperation is similar to that of any dog recovering from abdominal surgery. In a couple of weeks, he should be just fine.